Forced air furnaces for heating glass sheets in preparation for subsequent processing, such as tempering, are known in the art. For example, McMaster U.S. Pat. Nos. 4,529,380 and 4,505,671 disclose a glass sheet processing system which includes a heating furnace and a processing station for processing heated glass sheets to provide bending, tempering, bending and tempering, filming, etc. The furnace of U.S. Pat. Nos. 4,592,380 and 4,505,671 comprises an array of gas jets spaced above a conveyor within a heating chamber. The gas jets supply a primary gas flow directed toward the conveyor to provide forced convection heating of the glass sheets as the sheets are conveyed through the heating chamber. The gas jets of McMaster are arranged in linear series perpendicular to the length of the conveyor and the direction of travel of the glass sheets. Each series of jets is connected to a common linear supply manifold or conduit. Each supply conduit also extends widthwise in the heating furnace, perpendicular to the length of the conveyor. McMaster teaches that the array of gas jet pumps are spaced from each other transversely to the direction of conveyance so as to uniformly heat each conveyed glass sheet over its entire width.
Heating systems such as that described by McMaster appear to provide acceptable results for heating clear glass prior to tempering. Other known systems provide acceptable results for heating coated glass having an emissivity rating greater than about 0.2 prior to tempering. However, manufacturers have now begun to produce coated glass products having emissivity ratings in the range of 0.15-0.04. Prior art heating systems, including the system disclosed in U.S. Pat. Nos. 4,592,380 and 4,505,671, do not provide acceptable results for tempering glass having such low emissivity ratings.
Tamglass U.S. Pat. No. 5,951,734 discloses a system for heating low “e” glass sheeting, particularly low “e” glass sheeting having an emissivity rating below 0.2. One of the advantages provided by the heating system disclosed in Tamglass U.S. Pat. No. 5,951,734 is that it minimizes the problem of “oil-canning” and “bubbling” in glass sheeting caused by not heating the glass sheets uniformly.
Although the heating system disclosed in Tamglass U.S. Pat. No. 5,951,734 provides excellent results, it requires longer heating times to heat low “e” glass to the required temperature than the heating times required to heat non-coated glass. Therefore, it would be desirable to provide a system for heating low “e” glass which requires heating times that are shorter than the heating times required in the heating system of Tamglass U.S. Pat. No. 5,951,734 for heating low “e” glass to the required temperature.
Although heating systems that use nozzles to mix hot oven air and compressed air are known, these structures have been unsuccessful for heating low “e” glass. The nozzles of these prior systems produce a very small area of concentrated air flow, which damages the coating on the glass.